Touch based systems provide an intuitive way of interacting with digital content displayed on a display screen and touch detection capabilities are increasingly being introduced into mobile computing devices, such as personal digital assistants (PDAs), tablet PCs, digital music players and mobile telephones. Some of these devices require use of a particular stylus, however many devices allow use of any implement for touch interaction, including a user's finger. Whilst many devices are able to detect single touch events, distinguishing between multiple concurrent touch events is more complex and there are few commercially available devices with multi-touch capability.
Multi-touch systems have been demonstrated which use frustrated total internal reflection (FTIR) and an example system is shown in FIG. 1. A light emitting diode (LED) 101 is used to shine light into an acrylic pane 102 and this light undergoes total internal reflection (TIR) within the acrylic pane 102. When a finger is pressed against the top surface 103 of the acrylic pane 102, it causes light to be scattered. The scattered light passes through the rear surface 104 of the acrylic pane and can be detected by a camera 105 located behind the acrylic pane 102. Such touch sensing may be combined with a rear projection system which projects an image onto the rear surface of the acrylic pane.
Another technique for multi-touch detection which has been demonstrated uses a camera placed above a horizontal display surface. Image processing techniques are then used to determine when and where the user is touching the surface. Use of such a top-down configuration enables physical objects, such as documents or games pieces, in addition to hands and fingers, to be recognized. Such physical objects may not cause FTIR and therefore may not be recognized in an FTIR system or alternatively it may be features on the top surface of the object (i.e. the surface away from the acrylic pane) which may be used for recognition.